class Node():
    def __init__ (self,val):
        self.elem=val#当前节点
        self.next=None#下一节点
class List:
    def __init__(self):
        #头节点
        self.head = Node(-1)
    #前插法创建单链表
    def insert_before(self,data):
        for i in data:
            node=Node(i)#创建新节点
            #判断头节点的下一个节点是否为空
            if self.head.next is None: #如果为空则将新的节点加入到next
                self.head.next = node
            else:
                node.next = self.head.next#将头节点的下一节点加入当前节点的node
                self.head.next = node

#尾插法创建
def insert_tail(self,data):
    tail = self.head.next
    for i in data:
        node=Node(i)
        if tail is None:
            self.head.next = node
            tail = node
        else:
            tail.next = node
            tail = node

#打印单链表
def list_print(self):
    node = self.head.next
    while node:
        print(node.value,'',end='')
        node = node.next
    print('')
#删除链表中重复元素
def clear_repetition(self):
    cur=self.head
    while cur:
        while cur.next and cur.value==cur.next.value:
            cur.next=cur.next.next
        cur=cur.next



#第i个结点前插入值为value的结点
def list_element_add(self,i,value):

    node_new=Node(value)#创建新结点

    index=0

    node=self.head.next

    while node:#找位置
        index=index+1

        if index==i-1:
            break

        node=node.next

    if node is Node:
        return False

    node_new.next=node.next#插入结点
    node.next=node_new